1. Field of the Invention
The present invention relates to a method for separation and purification of high-purity 2,6-dimethylnaphthalene, more specifically to a method for separation and purification of 2,6-dimethylnaphthalene (hereinafter referred to as ‘2,6-DMN’), that separates high-purity 2,6-DMN from 1,5-dimethylnaphthalene (hereinafter referred to as ‘1,5-DMN’) produced from a dehydrogenation reaction of ortho-xylene(o-xylene; OX) and 1,5-butadiene (BD) as raw materials in a combination of isomerization and crystallization processes, thus to obtain high-purity 2,6-DMN in an industrially high yield.
2. Description of the Related Art
The 2,6-naphthalene dicarboxylic acid (hereinafter referred to as ‘2,6-NDA’) is used as a monomer for a polyethylene naphthalate (PEN) resin which is a high-functionality polyester polymer, or as a raw material for polymeric liquid crystals. Particularly, the PEN resin is known to be more excellent in heat resistance, tensile strength, impact strength and barrier property against gases such as oxygen, as compared to widely used polyethylene naphthalate (PEN) resins. Based on such the excellent physical properties, it is expected that the use of the PEN resins will be increased for the purpose of new applications and replacement of the conventional materials, and the demand of 2,6-NDA as a raw materials will be still increased.
There are a variety of the methods for preparation of 2,6-NDA as a raw material for a high-functionality PEN, among the methods, the method for oxidation of 2,6-NDA is a general and economical method. However, in the case of preparation of 2,6-NDA by oxidation of 2,6-DMN, the purity of 2,6-NDA would give great influence on the quality of the product. In particular, in the case where a trace amount of impurities is contained, the impurities would give great influence on the physical properties of the prepared 2,6-NDA, and on the polymerization process of PEN. Thus, for preparation of 2,6-NDA, other isomers (there exist ten isomers of dimethyl naphthalene (DMN) according to the positions two methyl groups) and high-purity 2,6-DMN which is free of impurities. Accordingly, it is required that 2,6-DMN should be also separated and purified at a high purity from a series of mixtures produced from the above-described reactions. Further, in order to easily improve the yield and the purity of separation and purification of 2,6-DMN, conversion to a high content of 2,6-DMN is required during the isomerization process.
Related arts known as the methods for obtaining high-purity and high-content 2,6-DMN can be largely classified into two categories.
One of them is a method for obtaining high-purity and high-content 2,6-DMN method by various separation and purification, and synthesis processes from the naphthalene isomer mixture contained in the liquid state materials which are generated upon the purification of coal or petroleum, and the processing.
The other method is a conventional method, which comprises performing an alkenylation reaction using OX and BD as starting materials in the presence of an alkali catalyst to obtain 5-ortho-tolyl-1-pentene (5-OTP), which is subjected to a cyclization reaction in the presence of a catalyst such as zeolite to obtain 1,5-dimethyltetralin (1,5-DMT), and subsequently, performing a dehydrogenation reaction using 1,5-DMT as a raw material and finally performing an isomerization reaction to obtain 2,6-DMN.
Japanese patent laid-open publication No. S49-134634 and U.S. Pat. No. 3,244,758 describe a method of obtaining o-tolylpentene-2 at a high yield by an alkenylation liquid state reaction using a catalyst such as alkali metals including K, Na or Na/K from OX and BD under the condition of 80 to 175° C., Japanese patent laid-open publication No. S50-8935 describes a method for preparing 1,5-DMT by subjecting o-tolylpentene-2 to cyclization, Japanese patent laid-open publication No. S48-76852 describes a method for preparing 1,5-DMN at a high yield and a high selectivity by subjecting 1,5-DMT to dehydration. Further, Japanese patent laid-open publication No. S50-129534 and U.S. Pat. No. 3,851,002 disclose a method for preparing an isomer mixture mainly consisting of 1,5-, 1,6- or 2,6-DMN by subjecting 1,5-DMN to isomerization, respectively.
Korean Patent No. 10-0469925 discloses a method for preparing 2,6-naphthalenecarboxylic acid ester, which comprises the steps of sequentially isomerizing a DMN mixture mainly consisting of dimethylnaphthalene (DMN) in the presence of a catalyst, crystallizing the isomerization product in the presence of a solvent, separating and recovering the precipitated crystals of 2,6-DMN, distilling the mother liquor after separation of the crystals to remove higher boiling point components and lower boiling point components as compared with DMN and to obtain a DMN residue, while recycling the DMN reside to an isomerization process to prepare a high-purity 2,6-DMN, and a method for preparing 2,6-naphthalenecarboxylic acid ester by liquid state oxidation of the 2,6-DMN obtained from the above-described method to prepare 2,6-NDA, and then esterifying the 2,6-NDA.
Hereinbelow, the conventional isomerization process for obtaining 2,6-DMN from the above-described DMN mixture will be specifically described.
U.S. Pat. No. 4,962,260 discloses a method for converting a DMN isomer mixture consisting of at least one of 1,5-, 1,6-, 1,7- and 1,8-DMNs with a batch or continuous mode liquid state reaction using a zeolite catalyst (which refers to an isomerization catalyst comprising a hydrogenation component of one 8 Group metal, to at least 25% by weight or more of 2,6-DMN isomers, wherein isomerization is performed, using a catalyst which has acid-treated USY zeolite having a SiO2/Al2O3 ratio of 4 to 10 and pore windows provided by 12-membered rings containing oxygen and a unit cell size of from about 24.2 to about 24.7 angstroms and at a reaction temperature in the range of from 200 to 400° C., a reaction pressure of 0.3 to 10 atm, and a weight hourly space velocity (WHSV) of 0.2 to 20 hr−1, and comprising inorganic materials such as a Group VIII metal, i.e., palladium, platinum or nickel, supported on a support material of alumina, silica-alumina, bentonite, and magnesia.
The invention disclosed in U.S. Pat. No. 4,783,569 is a gas state isomerization process comprising converting at least one 2,6-DMN isomer to a 2,6-triad having an equilibrium composition using a low-acidity, supported, and molecular-sieve-based catalyst, and simultaneously reducing the content of the isomers other than the 2,6-triad, and specifically, a process in which, as a catalyst, a supported crystalline borosilicate molecular sieve of supported crystalline aluminosilicate molecular sieve was used, and isomerization was performed under the condition of a ratio SiO2/Al2O3=50 to 150, and a reaction temperature of 250 to 400° C., 200 to 300° C. (in the case of a molecular sieve).
As such, the per se known methods for preparation of 2,6-DMN comprise separation of 2,6-DMN from a DMN isomer mixture by a distillation operation, or by using a catalyst, while the boiling points of the DMN isomers are close each other, and it is thus considerably difficult to perform separation and purification of 2,6-DMN.
Among the DMN isomers, 2,6-DMN has a highest melting point. Thus, 2,6-DMN can be separated and purified by a crystallization process. As such the method for separation of 2,6-DMN, there have been suggested a method by crystallization, a method by adsorption, a method for decomposing a complex by using a certain kind of an organic compound to form a complex with 2,6-DMN, and then separating the complex.
The related art involving crystallization separation and purification of 2,6-DMN will be described below.
Korean patent laid-open publication No. 2001-33746 suggests a method for preparation of 2,6-DMN at a high purity and a high yield, which comprises a series of steps of fraction, crystallization and adsorption from a DMN isomer mixture, without limiting the 2,6-DMN used in the preparation of PEN to certain isomers which are present in the supplied raw materials, the method involving, after performing the crystallization step, as a final purification step, dissolving the isomer mixture in p-, or o-xylene to perform adsorption and separation. Further, Japanese patent laid-open publication No. H9-301900 suggests a method for preparation of 2,6-DMN, which comprises performing crystallization from a DMN isomerization product to precipitate 2,6-DMN, from which a high-purity 2,6-DMN is obtained at a high yield, and a desirable, well-filterable 2,6-DMN crystals are precipitated, to industrially separate and recover a high-purity 2,6-DMN from the DMN isomerization product in the presence of a solvent by a process of a crystallization of 2,6-DMN. Examples of the solvent used for isomerization include pentene, hexane, heptane, octane, nonane, decane, undecane, dodecane, cyclopentane, cyclohexane, methylcyclopentane, methylcyclohexane, cyclooctane, methylcyclooctane, decane, methyldecane, and dimethyldecane. Japanese patent laid-open publication No. H9-249586 discloses a method for separation and recovery of 2,6-DMN, which is industrially favorable, and which comprises performing crystallization from a DMN isomer mixture to precipitate 2,6-DMN, from which a high-purity 2,6-DMN is obtained, and a long-term stability and a desired purity thereof can be maintained.
However, among the suggested, well-known methods, the method by crystallization is approved to be very simple and suitable as an industrial separation method, but it has a complicated process and a relatively low yield, and uses an expensive solvent, thus causing problems that relatively high fixed investment cost and high production cost are required. Upon reviewing the separation processes by crystallization, specific processes are not disclosed, but mostly they simply use a solvent to perform cooling for crystallization. Further, focus has been placed on the isomerization process or the adsorption process using a catalyst, rather than the crystallization process.